Process for preparing polyarylates

ABSTRACT

An improved process for preparing a polyarylate having a reduced viscosity of from about 0.1 to greater than 1.0 dl/gm comprising the following steps: 
     (a) reacting an acid anhydride derived from an acid containing from 2 to 8 carbon atoms with at least one dihydric phenol to form the corresponding diester; and 
     (b) reacting said diester with at least one aromatic dicarboxylic acid at a temperature sufficient to form the polyarylate, wherein the improvement comprises carrying out said process in the presence of a particular solid adsorbent.

BACKGROUND OF THE INVENTION

This invention is directed to an improved process for preparingpolyarylates.

Polyarylates are polyesters derived from a dihydric phenol, particularly2,2-bis(4-hydroxyphenyl)propane, also identified as Bisphenol-A, and anaromatic dicarboxylic acid, particularly mixtures of terephthalic andisophthalic acids. These polyarylates are high temperature, highperformance thermoplastic polymers with a good combination of thermaland mechanical properties. They also have good processability whichallows them to be molded into a variety of articles.

Many processes have been described in the literature for the preparationof polyarylates. One such process is the diacetate process. In thediacetate process, a dihydric phenol is converted to its diesterderivative, which is then reacted with an aromatic dicarboxylic acid(s)to form the polyarylate.

However, the polyarylate produced by the diacetate process have atendency to be colored which could make them unsuitable in some end useapplications. Thus there is a desire to improve the color of thepolyarylate for such end use applications.

Activated charcoal and other adsorbents have been used to remove coloror odor from liquids and gases. (V. R. Deitz, Bibliography of SolidAdsorbents, Washington, D.C., 1944; J. M. Brown, Revere Sugar Refinery,Charleston, Mass.). For example, extremely large quantities of fuller'searth and other adsorbents are used in purifying petroleum products (C.L. Mantell, "Adsorption," McGraw Hill, New York, 1945). However, the useof adsorbents such as activated charcoal to remove color from polymersis not seen reported in the literature. This may be due to the relativeineffectiveness of removing color from high molecular weight polymerswith adsorbents. The use of adsorbents during a polymerization reactionhas not been seen to be reported in the literature. There are extensiveliterature references which show the chemical reactivity of charcoal andother adsorbents. (Zechmeister, McNeely, and Solyom, J. Am. Chem. Soc.,64, 1922 (1942). For example, some charcoals are known oxidizing agentsat higher temperatures which would increase the color of most polymers.(E. Ledoux, "Vapor Adsorption, Industrial Applications and CompetingProcesses," Chemical Pub. Co., Brooklyn, 1945). In addition, physicaladsorption decreases with increasing temperature. Thus, the use ofadsorbents to improve the color of polyarylate is unexpected.

DESCRIPTION OF THE INVENTION

It has now been found that the use of from about 0.2 to about 2.0 wt.percent, based on the weight of the reaction mixture, of a particularadsorbent in the diacetate process to produce polyarylate polymersimproves the color of the resulting polymer.

The improved process for preparing a polyarylate having a reducedviscosity of from about 0.1 to greater than 1.0 dl/gm comprises thefollowing steps:

(a) reacting an acid anhydride derived from an acid containing from 2 to8 carbon atoms with at least one dihydric phenol to form thecorresponding diester; and

(b) reacting said diester with at least one aromatic dicarboxylic acidat a temperature sufficient to form the polyarylate, wherein theimprovement comprises carrying out said process in the presence of anadsorbent.

The adsorbents used herein include silica gel, activated charcoalderived from wood, paper pulp waste and coconut, and molecular seives,i.e., alumino silicates, and the like. The adsorbent may be added at anypoint in the polymerization process. Preferably, the adsorbent is addedat an early stage of the polymerization reaction.

The acid anhydride suitable for use herein is derived from an acidcontaining from 2 to 8 carbon atoms. The preferred acid anhydride isacetic anhydride.

Any dihydric phenol well known to those skilled in the art may be usedherein. Preferably, the dihydric phenol suitable for the use in thisinvention is of the following formula: ##STR1## wherein y isindependently selected from alkyl groups of 1 to 4 carbon atoms,chlorine or bromine, z independently has a value of from 0 to 4,inclusive, and R is independently selected from a divalent saturatedaliphatic hydrocarbon radical, particularly alkylene or alkylideneradicals having from 1 to 8 carbon atoms, especially C(CH₃)₂,cycloalkylene or cycloalkylidene radicals having up to and including 9carbon atoms, O, S, SO, SO₂, and CO, x is 0 or 1.

The dihydric phenols that may be used in this invention include thefollowing:

2,2-bis-(4-hydroxyphenyl)propane,

bis-(2-hydroxyphenyl)methane,

bis-(4-hydroxyphenyl)methane,

bis-(4-hydroxy-2,6-dimethyl-3-methoxyphenyl)methane,

1,1-bis-(4-hydroxyphenyl)ethane,

1,2-bis-(4-hydroxyphenyl)ethane,

1,1-bis-(4-hydroxy-2-chlorophenyl)ethane,

1,1-bis-(3-methyl-4-hydroxyphenyl)ethane,

1,3-bis-(3-methyl-4-hydroxyphenyl)propane

2,2-bis-(3-phenyl-4-hydroxyphenyl)propane,

2,2-bis-(3-isopropyl-4-hydroxyphenyl)propane,

2,2-bis-(2-isopropyl-4-hydroxyphenyl)propane,

2,2-bis-(4-hydroxyphenyl)pentane,

3,3-bis-(4-hydroxyphenyl)pentane,

2,2-bis-(4-hydroxyphenyl)heptane,

1,2-bis-(4-hydroxyphenyl)1,2-bis-(phenyl)-propane,

4,4'-(dihydroxyphenyl)ether,

4,4'-(dihydroxyphenyl)sulfide,

4,4'-(dihydroxyphenyl)sulfone,

4,4'-(dihydroxyphenyl)sulfoxide,

4,4'-(dihydroxybenzophenone),

hydroquinone, and

naphthalene diols.

Generally, the dihydric phenol reacts with the acid anhydride underconventional esterification conditions to form the dihydric phenoldiester. The reaction may take place in the presence or absence of asolvent. Additionally, the reaction may be conducted in the presence ofa conventional esterification catalyst or in the absence thereof.

The aromatic dicarboxylic acid(s) that may be used in this inventioninclude terephthalic acid, isophthalic acid, any of the naphthalenedicarboxylic acids and mixtures thereof, as well as alkyl substitutedhomologs of these carboxylic acids, wherein the alkyl group containsfrom 1 to about 4 carbon atoms, and acids containing other inertsubstituents such as halides, alkyl or aryl ethers, and the like.Preferably, mixtures of isophthalic and terephthalic acids are used. Theisophthalic acid to terephthalic acid ratio in the mixture is about20:80 to about 100:0, while the most preferred acid ratio is about 25:75to about 75:25. Also, from about 0.5 to about 20 percent of at least onealiphatic diacid containing from 2 to about 10 carbon atoms, such asadipic acid, sebacic acid, and the like, or mixtures thereof, may beadditionally used in the polymerization reaction.

The preparation of the polyarylate may be carried out in bulk preferablyin the presence of from about 10 to about 60, more preferably from about25 to about 60, and most preferably, from about 30 to about 60 weightpercent, based on the weight of the polyarylate produced, of an organicsolvent.

The preferred organic solvents are a diphenyl ether compound asdescribed in U.S. Pat. No. 4,294,956, a cycloaliphatic substitutedaromatic or heteroaromatic compound, as described in U.S. Pat. No.4,294,957, and a halogenated and/or etherated substituted aromatic orheteroaromatic compound, as described in U.S. Pat. No. 4,374,239, ormixtures of these.

The diphenyl ether compound, as described in U.S. Pat. No. 4,294,956,may be substituted. These substituents are selected from alkyl groups,chlorine, bromine or any substituent which does not interfere with thepolyarylate forming reaction or the reaction forming the diesterderivative of the dihydric phenol. Additionally, the diphenyl ethercompound may be used with up to 50 weight percent of other compounds,such as various biphenyls or any other compounds which do not interferewith the polyarylate forming reaction or the reaction forming thediester derivative of the dihydric phenol.

The cycloaliphatic compounds, or substituted aromatic or heteroaromaticcompounds, as described in U.S. Pat. No. 4,294,957, contain at least onebenzylic and/or tertiary hydrogen atoms. These compounds have a boilingpoint of about 150° to about 350° C., preferably from about 180° toabout 220° C., and a solubility parameter of ±4 within the solubilityparameter of the polyarylate being produced. Solubility parameter is ameasure for correlating polymer solvent interaction. It is defined in"Properties of Polymers," D. W. Van Krevelen, Elsevier ScientificPublishing Co., Amsterdam-Oxford-New York, 1976, pp. 141-155, as thesquare root of the cohesive energy density.

The cycloaliphatic compounds are of the following formulae: ##STR2##wherein R₁ is independently alkyl of 1 to 6 carbon atoms, cycloalkyl of6 to 18 carbon atoms, and a is an integer of 1 or 2.

The substituted aromatic compounds are of the following formula:##STR3## wherein R₂ is independently alkyl of 1 to 6 carbon atoms,cycloalkyl of 6 to 18 carbon atoms and aralkyl or alkaryl of 7 to 18carbon atoms and wherein the carbon atom of R₂ attached directly to thebenzene nucleus has 1 or 2 attached hydrogen atoms, and b is an integerof 1 to 6. ##STR4## wherein R₃ is independently alkyl of 1 to 6 carbonatoms, cycloalkyl of 6 to 18 carbon atoms, c is an integer of 1 or 2,and n is an integer of 1 to 6.

The heteroaromatic compounds are of the following formula: ##STR5##wherein A is S, O, or --CH═N--, R₄ is independently alkyl of 1 to 6carbon atoms, or cycloalkyl of 6 to 18 carbon atoms and wherein thecarbon atom of R₄ attached directly to the heteroaromatic nucleus has 1or 2 attached hydrogen atoms, and d is an integer of 1 to 4.

The preferred compounds encompased by structures (I) through (V) includexylenes, cumene, diethylbenzene, diisopropyl benzene,tetrahydronaphthalene or decahydronaphthalene.

Additionally, the cycloaliphatic, substituted aromatic or heteroaromaticcompounds may be used with up to 90 weight percent of other compoundssuch as diphenyl ether, dimethylsulfone, etc.

The halogenated and/or etherated substituted aromatic or heteroaromaticcompounds, as described in U.S. Pat. No. 4,374,239, are of formulae:##STR6## wherein X is independently Cl, Br, F, or OR₆, e is an integerof 1 to 6, R₅ is independently alkyl of 1 to 16 carbon atoms, cycloalkylof 6 to 18 carbon atoms, aryl of 6 to 18 carbon atoms, or aralkyl oralkaryl of 7 to 18 carbon atoms, R₆ is independently alkyl of 1 to 16carbon atoms or cycloalkyl of 6 to 18 carbon atoms, and f is integer of0 to (6-e). ##STR7## wherein X and R₅ are as previously defined, g is aninteger of 1 to 8 and h is an integer of 0 to (8-g).

The heteroaromatic compounds are of the following formula: ##STR8##wherein B is O, S, or --CH═N--, X and R₅ are as previously defined, j isan integer of 1 to 4 and k is integer of 0 to (4-j).

The compounds encompassed structures (VI) through (VIII) include1,2,3-trichlorobenzene; 1,2,4-trichlorobenzene, 1,2- or 1,3- or1,4-dichlorobenzene; 1,2,3- or 1,2,4- or 1,3,5-trimethoxybenzene; 1,2-or 1, or 1,4-dibromobenzene; chlorobenzene; bromobenzene;1-chloronaphthalene; 2-chloronaphthalene; 1-bromonaphthalene;2-bromonaphthalene; 1,2- or 1,3- or 1,4-dimethoxybenzene;2-bromotoluene; 2-chlorotoluene; 4-bromotoluene; 4-chlorotoluene;anisole; 2-methylanisole; 3-methylanisole; 4-methylanisole;2-chloroanisole; 3-chloroanisole; 4-chloroanisole; 2-bromoanisole;3-bromoanisole and 4-bromoanisole.

Additionally, the halogenated and/or etherated substituted aromatic orheteroaromatic compounds may be used with up to 90 weight percent ofother compounds such as diphenyl ether, dimethylsulfone, etc.

The amount of said solvents could vary during the polymerizationreaction. For example, it may be advantageous to increase progressivelythe amount of these solvents to maintain the reaction medium at constantviscosity.

A catalyst may be used to accelerate the rate of polyarylate formation.All the conventional catalysts capable of accelerating an ester exchangereaction are suitable for use herein. These include metal salts,generally the Group VII and VIII metal salts, such as magnesium,manganese or zinc salts. The salts are generally organic acid salts andinclude acetates, propionates, benzoates, oxalates, acetylacetonates, ormixtures thereof. A preferred catalyst is magnesium acetate. Thecatalyst is present in the reaction in a catalytically effective amountwhich can be, for example, from about 1 to about 1000, preferably fromabout 10 to about 50, parts per million, based on the weight of thepolyarylate produced.

The polyarylate polymer may be prepared in two reaction vessels byadding the acid anhydride and dihydric phenol to a reaction vessel andreacting these under esterification conditions described, supra to formthe diester derivative of the dihydric phenol. Residual acid anhydridemay then be removed by methods known in the art, such as by vacuumdistillation, or by chemical reaction with reactants which are notharmful to the polymerization, such as water, alcohols, dihydroxycompounds, and the like. The diester derivative may then be added to asecond reaction zone. Purification is not necessary. The second reactionzone contains aromatic dicarboxylic acid(s), and optionally solventand/or catalyst. The polymerization is then carried out. The activatedcharcoal may be added to the reaction zone either before or during thepolymerization reaction. Alternatively, the diester derivative is addedto the second reaction zone consisting of an aromatic dicarboxylicacid(s), and optionally, solvent and/or catalyst added thereto and thepolymerization carried out. Here again the adsorbent may be added at anytime during the polymerization. Any combination of adding the diesterderivative, aromatic dicarboxylic acid(s), and optionally solvent and/orcatalyst to a reaction vessel may be used.

In another embodiment of this invention, the diester derivative of thedihydric phenol is prepared in a reaction zone by reacting the acidanhydride and dihydric phenol therein under the esterificationconditions described, supra. Residual acid anhydride may then removed bythe procedures described, supra. Aromatic dicarboxylic acid(s) andoptionally solvent and/or catalyst is then added to the reaction zoneand the polymerization reaction carried out to produce the polyarylate.

The dihydric phenol diester is prepared by reacting a dihydric phenolwith an acid anhydride at a temperature of from about 130° to about 160°C. for a period of from about 0.5 to about 4 hours and at a pressure offrom about 1 to about 3 atmospheres. Generally, the reaction is carriedout using an excess of acid anhydride. The acid anhydride is used atabout 25 percent excess. The process is preferably carried out at apressure sufficient to have the mixture refluxing at the reactiontemperature. Under these conditions conversion to the dihydric phenoldiester is at least 99.9 percent.

The polymerization process is carried out at a temperature of from about260° to about 350° C. and preferably, from about 275° to about 295° C.The polymerization process is generally conducted in an inert atmosphere(such as argon or nitrogen) so that the oxygen content therein isminimized or eliminated. The oxygen content is generally less than about100, preferably less than about 30, and most preferably less than about10 parts per million. The process is preferably carried out at apressure sufficient to have the solvent refluxing at the reactiontemperature. This pressure is generally from about atmospheric to about11 atmospheres. Lower and higher pressures may also be used.

The polymerization reaction is conducted for a period of time sufficientto produce a polyarylate having a reduced viscosity of at least about0.1 to greater than 1.0 dl/gm, which time is generally less than about10 hours. The reaction time is generally in the range of from about 4hours to about 8 hours, depending on the particular polyarylate beingprepared.

The polymerization reaction of this invention may be carried outbatchwise or continuously and by using any apparatus desired.

The polyarylates may be prepared in the presence of one or morethermoplastic polymers.

The thermoplastic polymers suitable for use in this invention includeone or more of a polyester polymer, an aromatic polycarbonate, a styrenepolymer, an alkyl acrylate polymer, a polyurethane, a vinyl chloridepolymer, a poly(aryl ether), a copolyetherester block polymer, apolyhydroxyether, or combinations thereof, and the like.

The polyarylates produced by the process of this invention have areduced viscosity of from about 0.1 to greater than 1.0 dl/gm,preferably from about 0.2 to about 1.0 dl/gm as measured in chloroformor other solvents known in the art, such as para-chlorophenol,phenol/tetrachloroethane (60:40), and the like. Reduced viscosities ofthe polyarylates measured in these solvents generally have the samerange as those measured in chloroform.

The polyarylates may be prepared in the presence of materials such asmolecular weight regulators, antioxidants, and the like.

The polyarylates obtained by the process of this invention may be usedtogether with the well-known additives such as plasticizers, pigments,lubricating agents, mold release agents, stabilizers, inorganic fillers,and the like. These polyarylates may also further be blended with one ormore thermoplastic polymers such as polyesters, polycarbonates, styrenepolymers, alkyl acrylate polymers, polyurethanes, poly(aryl ether)polymers, polyamides, polyhydroxy ether polymers, copolyetherester blockcopolymers, polyamides, and the like.

EXAMPLES

The following examples serve to give specific illustrations of thepractice of this invention but they are not intended in any way to limitthe scope of this invention.

Preparation of polyarylate polymer

To a 250 ml 3-necked flask equipped with a nitrogen inlet, 30 cm vacuumjacketed column, mechanical stirrer, and a Teflon stir paddle was added8.31 g. of isophthalic acid, 8.31 g. of terephthalic acid, 31.39 g. ofbisphenol-A diacetate (prepared from bisphenol-A and acetic anhydride at150° C. and recrystallized from cyclohexane) and 23.87 g ofdiphenylether (distilled). The system was sparged with nitrogen for onehour at about 190 ml/min. The reaction mixture was then heated to 270°C. in one hour and kept at that temperature for an additional 6 or 11hours depending on whether lower molecular weights (reducedviscosity=0.3 dl/g) or higher molecular weight (reduced viscosity=0.6dl/g) was desired. The color factor was obtained by dissolving theentire reaction mixture in enough chloroform in order to make a 5 weightpercent solution, filtering out any contaminants, and measuring thesolution transmission of the polymer on a Fisher Electrophotometer(Model 81) at 425 mm. The color factor was determined by using acorrelation curve based on the comparison of the solution transmissionand the color factor determined on a plaque by a yellowness index basedon a chromaticity diagram derived in a manner similar to ASTM 1925 andthe book by G. Wyszecki and W. S. Stiles, "Color Science," John Wiley &Sons, Inc., New York, 1967. In some cases the polymer solution iscoagulated in methanol, dried, and redissolved in chloroform, filtered,and the solution transmission determined. No change in the transmissionoccurs during the coagulation step indicating that colored species arenot removed by this process.

Post Treatment of polyarylate with charcoal EXAMPLES 1 to 7

Polymers obtained by the process described in Preparation of polyarylatepolymer, supra, were dissolved in chloroform to form a 5 percentsolution and treated with various amounts of activated charcoal (DarcoG-60, 200 Mesh obtained from Matheson, Coleman & Bell) at roomtemperature (about 25° C.) and refluxing chloroform.

The results are shown in Table I.

                  TABLE I                                                         ______________________________________                                        Activated                                                                     Charcoal                     Color Factor                                     Example Charcoal (wt. %)                                                                          Time    (°C.)                                                                         Before                                                                              After                                ______________________________________                                        6 Hr Polymerizations - Reduced viscosity (RV) of 0.3 dl/g.                    Example 1:                                                                            0.1%        16 hrs  (25° C.)                                                                      132   77.8                                 Example 2                                                                             0.1%        16 hrs  (reflux)                                                                             132   77.8                                 Example 3                                                                             0.5%        30 min  (25° C.)                                                                      132   75.9                                 Example 4                                                                             1.0%        24 hrs  (25° C.)                                                                      104.3 63.8                                 Example 5                                                                             1.0%        24 hrs  (reflux)                                                                             34.2  13.7                                 11 Hr Polymerizations - Reduced viscosity of 0.6 dl/g.                        Example 6                                                                             0.5%        30 min  (25° C.)                                                                      235   219                                  Example 7                                                                             0.5%        30 min  (25° C.)                                                                      83.1  77.8                                 ______________________________________                                    

The data in the Table shows that activated charcoal treatments of lowermolecular weights polyarylates (RV=0.3 dl/g) is more effective thantreating a high molecular weight polyarylate (RV=0.6 dl/g).

EXAMPLES 8 to 17 Charcoal Treatments During Polymerization

The polymerizations in these Examples were carried out by the proceduredescribed in Preparation of Polyarylate Polymer, supra, except that theactivated charcoal (Examples 8 to 17) and silica gel (Example 18) wasadded together with the monomers, and the final polymer solution wasfiltered to remove the charcoal and silica gel, respectively, prior tothe measurement of solution transmission.

The results are shown in Table II.

                  TABLE II                                                        ______________________________________                                                 Activated        React.    Color                                     Example  Charcoal         Time(hrs.)                                                                              Factor                                    ______________________________________                                        Control A                                                                              None              6        132                                       Example 8                                                                              1.0% Darco G-60   6        22.5                                      Example 9                                                                              0.1% Darco G-60   6        48.2                                      Example 10                                                                             3% Coconut Charcoal.sup.1                                                                       6        60.4                                      Example 11                                                                             None             11        196                                       Example 12                                                                             1.0% Darco G-60  11        42.9                                      Example 13                                                                             3% Coconut Charcoa1.sup.2                                                                      11        39.5                                      Example 14                                                                             2% Nuchar.sup.3  11        60.4                                      Example 15                                                                             0.5% Nuchar      11        69.1                                      Example 16                                                                             0.2% Nuchar      11        85.0                                      Example 17                                                                             1.0% Nuchar      11        69.1                                      Example 18                                                                             2.0% silica gel.sup.4                                                                          11        41.0                                      ______________________________________                                         .sup.1 10 mesh, obtained from Matheson, Coleman and Bell (MCB)                .sup.2 10 mesh                                                                .sup.3 20 × 50 mesh activated charcoal obtained from Westvaco           Chemical Division                                                             .sup.4 grade 923, 100-200 mesh obtained from MCB                         

The data in Table II shows that adding the charcoal at the same time themonomers are added is more effective than post treatment of the polymerwith charcoal. Also, the data shows that effective decolorization isindependent of the particle size of the activated charcoal when added atthe beginning of the polymerization. The in situ charcoal treatment ismore effective than post-treating the high molecular weight polymer.

EXAMPLES 19 and 20

In these examples the polymerization was carried out by the proceduredescribed in the Preparation of polyarylate polymer, supra, except thatthe activated charcoal was added after the onset of polymerization.

The results are shown in Table III

                  TABLE III                                                       ______________________________________                                        Example  Activated Charcoal Color Factor                                      ______________________________________                                        Example 19                                                                             0.5% Nuchar, added 3 hrs                                                                          76                                                        after the temp reached 270° C.                                Example 20                                                                             0.5% Nuchar, added 4 hrs                                                                         159                                                        after the temp reached 270° C.                                ______________________________________                                    

Control B

The following were charged into an agitated one-gal 316 SS reactor andde-aerated with nitrogen for two hours:

1206 gms Bisphenol-A diacetate

319.2 gms isophthalic acid,

319.2 gms terephthalic acid,

918 gms diphenyl ether,

The mixture was heated to and held at 270° C. for about 6.5 hours.During this time, diphenyl ether was continuously pumped into thereactor and distillates taken overhead. Polymer concentration wasmaintained at an average of 58 percent.

The diphenyl ether feed was stopped and the reactor was pressurized tostop the distillation. Stabilizers were then added and the reactionmixture stirred for another 20 minutes. The polymer solution was thendischarged. Polymer weight average molecular weight was 38,000 by GPCusing polystyrene standard.

The results are shown in Table IV

EXAMPLE 21

The procedure of Control B was exactly repeated except that 55.3 gms. ofNuchar 20×50 mesh activated charcoal was added with the otheringredients. The results are shown in Table IV.

                  TABLE IV                                                        ______________________________________                                        Example     Activated Charcoal                                                                          Color Factor                                        ______________________________________                                        Control B   None          103                                                 Example 21  2 percent Nuchar                                                                             37                                                 ______________________________________                                    

What is claimed is:
 1. An improved process for preparing a polyarylatehaving a reduced viscosity of from about 0.1 to greater than 1.0 dl/gwhich process comprises the following steps:(a) reacting an acidanhydride derived from an acid containing from 2 to 8 carbon atoms withat least one dihydric phenol to form the corresponding diester; and (b)reacting said diester with at least one aromatic dicarboxylic acid at atemperature sufficient to form the polyarylate, wherein the improvementcomprises carrying out said process in the presence of from about 0.2 toabout 2.0 weight percent, based on the weight of the reaction mixture ofan adsorbent selected from silica gel, activated charcoal or a molecularsieve.
 2. A process as defined in claim 1 wherein the adsorbent isactivated charcoal.
 3. A process as defined in claim 1, wherein the acidanhydride is acetic anhydride.
 4. A process as defined in claim 1,wherein the dihydric phenol is of the following formula: ##STR9##wherein y is independently selected from alkyl groups of 1 to 4 carbonatoms, chlorine or bromine, z independently has a value of from 0 to 4,inclusive, R is independently selected from a divalent saturatedhydrocarbon radical having 1 to 8 carbon atoms, a cycloalkylene orcycloalkylidene radical having up to and including 9 carbon atoms, O, S,SO, SO₂, CO, x is 0 or
 1. 5. A process as defined in claim 1, 3 or 4wherein the dihydric phenol is bisphenol-A.
 6. A process as defined inclaim 1 wherein the isophthalic acid to terephthalic acid ration in themixture of acids is about 20:80 to about 100:0.
 7. A process as definedin claim 6, wherein the ratio is about 25:75 to about 75:25.
 8. Aprocess as defined in claim 1, wherein an aliphatic diacid containingfrom 2 to about 10 carbon atoms is added in step (b).
 9. A process asdefined in claim 8, wherein the diacid is selected from adipic acid orsebacic acid, or mixtures thereof.
 10. A process as defined in claim 8or 9, wherein from about 0.5 to about 20 percent of the diacid is added.11. A process as defined in claim 1, wherein the diester is reacted withat least one aromatic dicarboxylic acid in the presence of an organicsolvent.
 12. A process as defined in claim 11, wherein the solvent isselected from a diphenyl ether compound, a cycloaliphatic compound or asubstituted aromatic or heteroaromatic compound, or a halogenated and/oretherated substituted aromatic or heteroaromatic compound, or mixturesthereof.
 13. A process as defined in claim 11, wherein the solvent isdiphenyl ether.
 14. A process as defined in claim 11 or 12, or 13,wherein the solvent is present in an amount of from about 10 to about 60weight percent, based on the weight of the polyarylate polymer produced.15. A process as defined in claim 11, wherein the solvent is present inan amount of from about 25 to about 60 weight percent, based on theweight of the polyarylate polymer produced.
 16. A process as defined inclaim 1, wherein the reaction in step (b) is carried out in the presenceof a catalytically effective amount of a catalyst.
 17. A process asdefined in claim 16, wherein the catalyst is selected from the Group VIIor Group VIII metal salts.
 18. A process as defined in claim 17, whereinthe salts are magnesium, manganese, or zinc salts.
 19. A process asdefined in claim 17 or 18, wherein the catalyst is magnesium acetate.20. A process as defined in claim 1 wherein the temperature is fromabout 260° to about 350° C.
 21. A process as defined in claim 1 or 20,wherein the temperature is from about 275° to about 295° C.
 22. Aprocess as defined in claim 1 wherein the process is carried out in thepresence of a thermoplastic polymer.
 23. A process as defined in claim22 wherein the thermoplastic polymer is selected from a polyester, anaromatic polycarbonate, a styrene polymer, an allyl acrylate polymer, apolyurethane, a vinyl chloride polymer, a poly(aryl ether), acopolyetherester block polymer, a poly hydroxyether, or mixturesthereof.
 24. A process as in claim 1 wherein the adsorbent is silicagel.